PERLHACK(1) Perl Programmers Reference Guide PERLHACK(1)

PERLHACK(1) Perl Programmers Reference Guide PERLHACK(1) #

PERLHACK(1) Perl Programmers Reference Guide PERLHACK(1)

NNAAMMEE #

 perlhack - How to hack on Perl

DDEESSCCRRIIPPTTIIOONN #

 This document explains how Perl development works.  It includes details
 about the Perl 5 Porters email list, the Perl repository, the Perl bug
 tracker, patch guidelines, and commentary on Perl development philosophy.

SSUUPPEERR QQUUIICCKK PPAATTCCHH GGUUIIDDEE #

 If you just want to submit a single small patch like a pod fix, a test
 for a bug, comment fixes, etc., it's easy! Here's how:

 •   Check out the source repository

     The perl source is in a git repository.  You can clone the repository
     with the following command:

       % git clone https://github.com/Perl/perl5.git perl

 •   Ensure you're following the latest advice

     In case the advice in this guide has been updated recently, read the
     latest version directly from the perl source:

       % perldoc pod/perlhack.pod

 •   Create a branch for your change

     Create a branch based on blead to commit your change to, which will
     later be used to send it to the Perl issue tracker.

       % git checkout -b mychange

 •   Make your change

     Hack, hack, hack.  Keep in mind that Perl runs on many different
     platforms, with different operating systems that have different
     capabilities, different filesystem organizations, and even different
     character sets.  perlhacktips gives advice on this.

 •   Test your change

     You can run all the tests with the following commands:

       % ./Configure -des -Dusedevel
       % make test

     Keep hacking until the tests pass.

 •   Commit your change

     Committing your work will save the change _o_n _y_o_u_r _l_o_c_a_l _s_y_s_t_e_m:

       % git commit -a -m 'Commit message goes here'

     Make sure the commit message describes your change in a single
     sentence.  For example, "Fixed spelling errors in perlhack.pod".

 •   Send your change to the Perl issue tracker

     The next step is to submit your patch to the Perl core ticket system.

     Create a GitHub fork of the perl5 repository and add it as a remote,
     if you haven't already, as described in the GitHub documentation at
     <https://help.github.com/en/articles/working-with-forks>.

       % git remote add fork git@github.com:MyUser/perl5.git

     For more information, see "Connecting to GitHub with SSH"
     <https://docs.github.com/en/free-pro-
     team@latest/github/authenticating-to-github/connecting-to-github-
     with-ssh>.

     If you'd rather use an HTTPS URL for your "git push" see "Cloning
     with HTTPS URLs" <https://docs.github.com/en/free-pro-
     team@latest/github/using-git/which-remote-url-should-i-use#cloning-
     with-https-urls>.

       % git remote add fork https://github.com/MyUser/perl5.git

     Then, push your new branch to your fork.

       % git push -u fork mychange

     Finally, create a Pull Request on GitHub from your branch to blead as
     described in the GitHub documentation at
     <https://help.github.com/en/articles/creating-a-pull-request-from-a-fork>.

 •   Thank you

     The porters appreciate the time you spent helping to make Perl
     better.  Thank you!

 •   Acknowledgement

     All contributors are credited (by name and email address) in the
     AUTHORS file, which is part of the perl distribution, as well as the
     Git commit history.

     If you don’t want to be included in the AUTHORS file, just let us
     know. Otherwise we will take your submission of a patch as permission
     to credit you in the AUTHORS file.

 •   Next time

     The next time you wish to make a patch, you need to start from the
     latest perl in a pristine state.  Check you don't have any local
     changes or added files in your perl check-out which you wish to keep,
     then run these commands:

       % git checkout blead
       % git pull
       % git reset --hard origin/blead
       % git clean -dxf

BBUUGG RREEPPOORRTTIINNGG #

 If you want to report a bug in Perl, or browse existing Perl bugs and
 patches, use the GitHub issue tracker at
 <https://github.com/perl/perl5/issues>.

 Please check the archive of the perl5-porters list (see below) and/or the
 bug tracking system before submitting a bug report.  Often, you'll find
 that the bug has been reported already.

 You can log in to the bug tracking system and comment on existing bug
 reports.  If you have additional information regarding an existing bug,
 please add it.  This will help the porters fix the bug.

PPEERRLL 55 PPOORRTTEERRSS #

 The perl5-porters (p5p) mailing list is where the Perl standard
 distribution is maintained and developed.  The people who maintain Perl
 are also referred to as the "Perl 5 Porters", "p5p" or just the
 "porters".

 A searchable archive of the list is available at
 <https://markmail.org/search/?q=perl5-porters>.  There is also an archive
 at <https://archive.develooper.com/perl5-porters@perl.org/>.

ppeerrll--cchhaannggeess mmaaiilliinngg lliisstt The perl5-changes mailing list receives a copy of each patch that gets submitted to the maintenance and development branches of the perl repository. See https://lists.perl.org/list/perl5-changes.html for subscription and archive information.

##pp55pp oonn IIRRCC Many porters are also active on the irc://irc.perl.org/#p5p channel. Feel free to join the channel and ask questions about hacking on the Perl core.

GGEETTTTIINNGG TTHHEE PPEERRLL SSOOUURRCCEE #

 All of Perl's source code is kept centrally in a Git repository at
 _g_i_t_h_u_b_._c_o_m.  The repository contains many Perl revisions from Perl 1
 onwards and all the revisions from Perforce, the previous version control
 system.

 For much more detail on using git with the Perl repository, please see
 perlgit.

RReeaadd aacccceessss vviiaa GGiitt You will need a copy of Git for your computer. You can fetch a copy of the repository using the git protocol:

   % git clone git@github.com:Perl/perl5.git perl

 This clones the repository and makes a local copy in the _p_e_r_l directory.

 If you cannot use the git protocol for firewall reasons, you can also
 clone via http:

   % git clone https://github.com/Perl/perl5.git perl

RReeaadd aacccceessss vviiaa tthhee wweebb You may access the repository over the web. This allows you to browse the tree, see recent commits, subscribe to repository notifications, search for particular commits and more. You may access it at https://github.com/Perl/perl5.

WWrriittee aacccceessss vviiaa ggiitt If you have a commit bit, please see perlgit for more details on using git.

PPAATTCCHHIINNGG PPEERRLL #

 If you're planning to do more extensive work than a single small fix, we
 encourage you to read the documentation below.  This will help you focus
 your work and make your patches easier to incorporate into the Perl
 source.

SSuubbmmiittttiinngg ppaattcchheess If you have a small patch to submit, please submit it via the GitHub Pull Request workflow. You may also send patches to the p5p list.

 Patches are reviewed and discussed on GitHub or the p5p list.  Simple,
 uncontroversial patches will usually be applied without any discussion.
 When the patch is applied, the ticket will be updated and you will
 receive email.

 In other cases, the patch will need more work or discussion.  You are
 encouraged to participate in the discussion and advocate for your patch.
 Sometimes your patch may get lost in the shuffle.  It's appropriate to
 send a reminder email to p5p if no action has been taken in a month.
 Please remember that the Perl 5 developers are all volunteers, and be
 polite.

 Changes are always applied directly to the main development branch,
 called "blead".  Some patches may be backported to a maintenance branch.
 If you think your patch is appropriate for the maintenance branch (see
 "MAINTENANCE BRANCHES" in perlpolicy), please explain why when you submit
 it.

GGeettttiinngg yyoouurr ppaattcchh aacccceepptteedd If you are submitting a code patch there are several things that you can do to help the Perl 5 Porters accept your patch.

 _P_a_t_c_h _s_t_y_l_e

 Using the GitHub Pull Request workflow, your patch will automatically be
 available in a suitable format.  If you wish to submit a patch to the p5p
 list for review, make sure to create it appropriately.

 If you used git to check out the Perl source, then using "git
 format-patch" will produce a patch in a style suitable for Perl.  The
 "format-patch" command produces one patch file for each commit you made.
 If you prefer to send a single patch for all commits, you can use "git
 diff".

   % git checkout blead
   % git pull
   % git diff blead my-branch-name

 This produces a patch based on the difference between blead and your
 current branch.  It's important to make sure that blead is up to date
 before producing the diff, that's why we call "git pull" first.

 We strongly recommend that you use git if possible.  It will make your
 life easier, and ours as well.

 However, if you're not using git, you can still produce a suitable patch.
 You'll need a pristine copy of the Perl source to diff against.  The
 porters prefer unified diffs.  Using GNU "diff", you can produce a diff
 like this:

   % diff -Npurd perl.pristine perl.mine

 Make sure that you "make realclean" in your copy of Perl to remove any
 build artifacts, or you may get a confusing result.

 _C_o_m_m_i_t _m_e_s_s_a_g_e

 As you craft each patch you intend to submit to the Perl core, it's
 important to write a good commit message.  This is especially important
 if your submission will consist of a series of commits.

 The first line of the commit message should be a short description
 without a period.  It should be no longer than the subject line of an
 email, 50 characters being a good rule of thumb.

 A lot of Git tools (Gitweb, GitHub, git log --pretty=oneline, ...) will
 only display the first line (cut off at 50 characters) when presenting
 commit summaries.

 The commit message should include a description of the problem that the
 patch corrects or new functionality that the patch adds.

 As a general rule of thumb, your commit message should help a programmer
 who knows the Perl core quickly understand what you were trying to do,
 how you were trying to do it, and why the change matters to Perl.

 •   Why

     Your commit message should describe why the change you are making is
     important.  When someone looks at your change in six months or six
     years, your intent should be clear.

     If you're deprecating a feature with the intent of later simplifying
     another bit of code, say so.  If you're fixing a performance problem
     or adding a new feature to support some other bit of the core,
     mention that.

 •   What

     Your commit message should describe what part of the Perl core you're
     changing and what you expect your patch to do.

 •   How

     While it's not necessary for documentation changes, new tests or
     trivial patches, it's often worth explaining how your change works.
     Even if it's clear to you today, it may not be clear to a porter next
     month or next year.

 A commit message isn't intended to take the place of comments in your
 code.  Commit messages should describe the change you made, while code
 comments should describe the current state of the code.

 If you've just implemented a new feature, complete with doc, tests and
 well-commented code, a brief commit message will often suffice.  If,
 however, you've just changed a single character deep in the parser or
 lexer, you might need to write a small novel to ensure that future
 readers understand what you did and why you did it.

 _C_o_m_m_e_n_t_s_, _C_o_m_m_e_n_t_s_, _C_o_m_m_e_n_t_s

 Be sure to adequately comment your code.  While commenting every line is
 unnecessary, anything that takes advantage of side effects of operators,
 that creates changes that will be felt outside of the function being
 patched, or that others may find confusing should be documented.  If you
 are going to err, it is better to err on the side of adding too many
 comments than too few.

 The best comments explain _w_h_y the code does what it does, not _w_h_a_t _i_t
 _d_o_e_s.

 _S_t_y_l_e

 In general, please follow the particular style of the code you are
 patching.

 In particular, follow these general guidelines for patching Perl sources:

 •   4-wide indents for code, 2-wide indents for nested CPP "#define"s,
     with 8-wide tabstops.

 •   Use spaces for indentation, not tab characters.

     The codebase is a mixture of tabs and spaces for indentation, and we
     are moving to spaces only.  Converting lines you're patching from
     8-wide tabs to spaces will help this migration.

 •   Try not to exceed 79 columns

     In general, we target 80 column lines.  When sticking to 80 columns
     would lead to torturous code or rework, it's fine to go longer.  Try
     to keep your excess past 80 to a minimum.

 •   ANSI C prototypes

 •   Uncuddled elses and "K&R" style for indenting control constructs

 •   No C++ style (//) comments

 •   Mark places that need to be revisited with XXX (and revisit often!)

 •   Opening brace lines up with "if" when conditional spans multiple
     lines; should be at end-of-line otherwise

 •   In function definitions, name starts in column 0 (return value-type
     is on previous line)

 •   Single space after keywords that are followed by parens, no space
     between function name and following paren

 •   Avoid assignments in conditionals, but if they're unavoidable, use
     extra paren, e.g. "if (a && (b = c)) ..."

 •   "return foo;" rather than "return(foo);"

 •   "if (!foo) ..." rather than "if (foo == FALSE) ..." etc.

 •   Do not declare variables using "register".  It may be
     counterproductive with modern compilers, and is deprecated in C++,
     under which the Perl source is regularly compiled.

 •   In-line functions that are in headers that are accessible to XS code
     need to be able to compile without warnings with commonly used extra
     compilation flags, such as gcc's "-Wswitch-default" which warns
     whenever a switch statement does not have a "default" case.  The use
     of these extra flags is to catch potential problems in legal C code,
     and is often used by Perl aggregators, such as Linux distributors.

 _T_e_s_t _s_u_i_t_e

 If your patch changes code (rather than just changing documentation), you
 should also include one or more test cases which illustrate the bug
 you're fixing or validate the new functionality you're adding.  In
 general, you should update an existing test file rather than create a new
 one.

 Your test suite additions should generally follow these guidelines
 (courtesy of Gurusamy Sarathy <gsar@activestate.com>):

 •   Know what you're testing.  Read the docs, and the source.

 •   Tend to fail, not succeed.

 •   Interpret results strictly.

 •   Use unrelated features (this will flush out bizarre interactions).

 •   Use non-standard idioms (otherwise you are not testing TIMTOWTDI).

 •   Avoid using hardcoded test numbers whenever possible (the
     EXPECTED/GOT found in t/op/tie.t is much more maintainable, and gives
     better failure reports).

 •   Give meaningful error messages when a test fails.

 •   Avoid using qx// and ssyysstteemm(()) unless you are testing for them.  If
     you do use them, make sure that you cover _all_ perl platforms.

 •   Unlink any temporary files you create.

 •   Promote unforeseen warnings to errors with $SIG{__WARN__}.

 •   Be sure to use the libraries and modules shipped with the version
     being tested, not those that were already installed.

 •   Add comments to the code explaining what you are testing for.

 •   Make updating the '1..42' string unnecessary.  Or make sure that you
     update it.

 •   Test _all_ behaviors of a given operator, library, or function.

     Test all optional arguments.

     Test return values in various contexts (boolean, scalar, list,
     lvalue).

     Use both global and lexical variables.

     Don't forget the exceptional, pathological cases.

PPaattcchhiinngg aa ccoorree mmoodduullee This works just like patching anything else, with one extra consideration.

 Modules in the _c_p_a_n_/ directory of the source tree are maintained outside
 of the Perl core.  When the author updates the module, the updates are
 simply copied into the core.  See that module's documentation or its
 listing on <https://metacpan.org/> for more information on reporting bugs
 and submitting patches.

 In most cases, patches to modules in _c_p_a_n_/ should be sent upstream and
 should not be applied to the Perl core individually.  If a patch to a
 file in _c_p_a_n_/ absolutely cannot wait for the fix to be made upstream,
 released to CPAN and copied to blead, you must add (or update) a
 "CUSTOMIZED" entry in the _P_o_r_t_i_n_g_/_M_a_i_n_t_a_i_n_e_r_s_._p_l file to flag that a
 local modification has been made.  See _P_o_r_t_i_n_g_/_M_a_i_n_t_a_i_n_e_r_s_._p_l for more
 details.

 In contrast, modules in the _d_i_s_t_/ directory are maintained in the core.

UUppddaattiinngg ppeerrllddeellttaa For changes significant enough to warrant a _p_o_d_/_p_e_r_l_d_e_l_t_a_._p_o_d entry, the porters will greatly appreciate it if you submit a delta entry along with your actual change. Significant changes include, but are not limited to:

 •   Adding, deprecating, or removing core features

 •   Adding, deprecating, removing, or upgrading core or dual-life modules

 •   Adding new core tests

 •   Fixing security issues and user-visible bugs in the core

 •   Changes that might break existing code, either on the perl or C level

 •   Significant performance improvements

 •   Adding, removing, or significantly changing documentation in the _p_o_d_/
     directory

 •   Important platform-specific changes

 Please make sure you add the perldelta entry to the right section within
 _p_o_d_/_p_e_r_l_d_e_l_t_a_._p_o_d.  More information on how to write good perldelta
 entries is available in the "Style" section of
 _P_o_r_t_i_n_g_/_h_o_w___t_o___w_r_i_t_e___a___p_e_r_l_d_e_l_t_a_._p_o_d.

WWhhaatt mmaakkeess ffoorr aa ggoooodd ppaattcchh?? New features and extensions to the language can be contentious. There is no specific set of criteria which determine what features get added, but here are some questions to consider when developing a patch:

 _D_o_e_s _t_h_e _c_o_n_c_e_p_t _m_a_t_c_h _t_h_e _g_e_n_e_r_a_l _g_o_a_l_s _o_f _P_e_r_l_?

 Our goals include, but are not limited to:

 1.  Keep it fast, simple, and useful.

 2.  Keep features/concepts as orthogonal as possible.

 3.  No arbitrary limits (platforms, data sizes, cultures).

 4.  Keep it open and exciting to use/patch/advocate Perl everywhere.

 5.  Either assimilate new technologies, or build bridges to them.

 _W_h_e_r_e _i_s _t_h_e _i_m_p_l_e_m_e_n_t_a_t_i_o_n_?

 All the talk in the world is useless without an implementation.  In
 almost every case, the person or people who argue for a new feature will
 be expected to be the ones who implement it.  Porters capable of coding
 new features have their own agendas, and are not available to implement
 your (possibly good) idea.

 _B_a_c_k_w_a_r_d_s _c_o_m_p_a_t_i_b_i_l_i_t_y

 It's a cardinal sin to break existing Perl programs.  New warnings can be
 contentious--some say that a program that emits warnings is not broken,
 while others say it is.  Adding keywords has the potential to break
 programs, changing the meaning of existing token sequences or functions
 might break programs.

 The Perl 5 core includes mechanisms to help porters make backwards
 incompatible changes more compatible such as the feature and deprecate
 modules.  Please use them when appropriate.

 _C_o_u_l_d _i_t _b_e _a _m_o_d_u_l_e _i_n_s_t_e_a_d_?

 Perl 5 has extension mechanisms, modules and XS, specifically to avoid
 the need to keep changing the Perl interpreter.  You can write modules
 that export functions, you can give those functions prototypes so they
 can be called like built-in functions, you can even write XS code to mess
 with the runtime data structures of the Perl interpreter if you want to
 implement really complicated things.

 Whenever possible, new features should be prototyped in a CPAN module
 before they will be considered for the core.

 _I_s _t_h_e _f_e_a_t_u_r_e _g_e_n_e_r_i_c _e_n_o_u_g_h_?

 Is this something that only the submitter wants added to the language, or
 is it broadly useful?  Sometimes, instead of adding a feature with a
 tight focus, the porters might decide to wait until someone implements
 the more generalized feature.

 _D_o_e_s _i_t _p_o_t_e_n_t_i_a_l_l_y _i_n_t_r_o_d_u_c_e _n_e_w _b_u_g_s_?

 Radical rewrites of large chunks of the Perl interpreter have the
 potential to introduce new bugs.

 _H_o_w _b_i_g _i_s _i_t_?

 The smaller and more localized the change, the better.  Similarly, a
 series of small patches is greatly preferred over a single large patch.

 _D_o_e_s _i_t _p_r_e_c_l_u_d_e _o_t_h_e_r _d_e_s_i_r_a_b_l_e _f_e_a_t_u_r_e_s_?

 A patch is likely to be rejected if it closes off future avenues of
 development.  For instance, a patch that placed a true and final
 interpretation on prototypes is likely to be rejected because there are
 still options for the future of prototypes that haven't been addressed.

 _I_s _t_h_e _i_m_p_l_e_m_e_n_t_a_t_i_o_n _r_o_b_u_s_t_?

 Good patches (tight code, complete, correct) stand more chance of going
 in.  Sloppy or incorrect patches might be placed on the back burner until
 fixes can be made, or they might be discarded altogether without further
 notice.

 _I_s _t_h_e _i_m_p_l_e_m_e_n_t_a_t_i_o_n _g_e_n_e_r_i_c _e_n_o_u_g_h _t_o _b_e _p_o_r_t_a_b_l_e_?

 The worst patches make use of system-specific features.  It's highly
 unlikely that non-portable additions to the Perl language will be
 accepted.

 _I_s _t_h_e _i_m_p_l_e_m_e_n_t_a_t_i_o_n _t_e_s_t_e_d_?

 Patches which change behaviour (fixing bugs or introducing new features)
 must include regression tests to verify that everything works as
 expected.

 Without tests provided by the original author, how can anyone else
 changing perl in the future be sure that they haven't unwittingly broken
 the behaviour the patch implements? And without tests, how can the
 patch's author be confident that his/her hard work put into the patch
 won't be accidentally thrown away by someone in the future?

 _I_s _t_h_e_r_e _e_n_o_u_g_h _d_o_c_u_m_e_n_t_a_t_i_o_n_?

 Patches without documentation are probably ill-thought out or incomplete.
 No features can be added or changed without documentation, so submitting
 a patch for the appropriate pod docs as well as the source code is
 important.

 _I_s _t_h_e_r_e _a_n_o_t_h_e_r _w_a_y _t_o _d_o _i_t_?

 Larry said "Although the Perl Slogan is _T_h_e_r_e_'_s _M_o_r_e _T_h_a_n _O_n_e _W_a_y _t_o _D_o
 _I_t, I hesitate to make 10 ways to do something".  This is a tricky
 heuristic to navigate, though--one man's essential addition is another
 man's pointless cruft.

 _D_o_e_s _i_t _c_r_e_a_t_e _t_o_o _m_u_c_h _w_o_r_k_?

 Work for the committers, work for Perl programmers, work for module
 authors, ... Perl is supposed to be easy.

 _P_a_t_c_h_e_s _s_p_e_a_k _l_o_u_d_e_r _t_h_a_n _w_o_r_d_s

 Working code is always preferred to pie-in-the-sky ideas.  A patch to add
 a feature stands a much higher chance of making it to the language than
 does a random feature request, no matter how fervently argued the request
 might be.  This ties into "Will it be useful?", as the fact that someone
 took the time to make the patch demonstrates a strong desire for the
 feature.

TTEESSTTIINNGG #

 The core uses the same testing style as the rest of Perl, a simple
 "ok/not ok" run through Test::Harness, but there are a few special
 considerations.

 There are three ways to write a test in the core: Test::More, _t_/_t_e_s_t_._p_l
 and ad hoc "print $test ? "ok 42\n" : "not ok 42\n"".  The decision of
 which to use depends on what part of the test suite you're working on.
 This is a measure to prevent a high-level failure (such as Config.pm
 breaking) from causing basic functionality tests to fail.

 The _t_/_t_e_s_t_._p_l library provides some of the features of Test::More, but
 avoids loading most modules and uses as few core features as possible.

 If you write your own test, use the Test Anything Protocol
 <https://testanything.org>.

 •   _t_/_b_a_s_e, _t_/_c_o_m_p and _t_/_o_p_b_a_s_i_c

     Since we don't know if "require" works, or even subroutines, use ad
     hoc tests for these three.  Step carefully to avoid using the feature
     being tested.  Tests in _t_/_o_p_b_a_s_i_c, for instance, have been placed
     there rather than in _t_/_o_p because they test functionality which
     _t_/_t_e_s_t_._p_l presumes has already been demonstrated to work.

 •   All other subdirectories of _t_/

     Now that basic rreeqquuiirree(()) and subroutines are tested, you can use the
     _t_/_t_e_s_t_._p_l library.

     You can also use certain libraries like Config conditionally, but be
     sure to skip the test gracefully if it's not there.

 •   Test files not found under _t_/

     This category includes _._t files underneath directories such as _d_i_s_t,
     _e_x_t and _l_i_b.  Since the core of Perl has now been tested, Test::More
     can and now should be used.  You can also use the full suite of core
     modules in the tests.  (As noted in "Patching a core module" above,
     changes to _._t files found under _c_p_a_n_/ should be submitted to the
     upstream maintainers of those modules.)

 When you say "make test", Perl uses the _t_/_T_E_S_T program to run the test
 suite (except under Win32 where it uses _t_/_h_a_r_n_e_s_s instead).  All tests
 are run from the _t_/ directory, nnoott the directory which contains the test.
 This causes some problems with the tests in _l_i_b_/, so here's some
 opportunity for some patching.

 You must be triply conscious of cross-platform concerns.  This usually
 boils down to using File::Spec, avoiding things like "fork()" and
 "system()" unless absolutely necessary, and not assuming that a given
 character has a particular ordinal value (code point) or that its UTF-8
 representation is composed of particular bytes.

 There are several functions available to specify characters and code
 points portably in tests.  The always-preloaded functions
 "utf8::unicode_to_native()" and its inverse "utf8::native_to_unicode()"
 take code points and translate appropriately.  The file
 _t_/_c_h_a_r_s_e_t___t_o_o_l_s_._p_l has several functions that can be useful.  It has
 versions of the previous two functions that take strings as inputs -- not
 single numeric code points: "uni_to_native()" and "native_to_uni()".  If
 you must look at the individual bytes comprising a UTF-8 encoded string,
 "byte_utf8a_to_utf8n()" takes as input a string of those bytes encoded
 for an ASCII platform, and returns the equivalent string in the native
 platform.  For example, "byte_utf8a_to_utf8n("\xC2\xA0")" returns the
 byte sequence on the current platform that form the UTF-8 for "U+00A0",
 since "\xC2\xA0" are the UTF-8 bytes on an ASCII platform for that code
 point.  This function returns "\xC2\xA0" on an ASCII platform, and
 "\x80\x41" on an EBCDIC 1047 one.

 But easiest is, if the character is specifiable as a literal, like "A" or
 "%", to use that; if not so specificable, you can use "\N{}" , if the
 side effects aren't troublesome.  Simply specify all your characters in
 hex, using "\N{U+ZZ}" instead of "\xZZ".  "\N{}" is the Unicode name, and
 so it always gives you the Unicode character.  "\N{U+41}" is the
 character whose Unicode code point is 0x41, hence is 'A' on all
 platforms.  The side effects are:

 •   These select Unicode rules.  That means that in double-quotish
     strings, the string is always converted to UTF-8 to force a Unicode
     interpretation (you can "utf8::downgrade()" afterwards to convert
     back to non-UTF8, if possible).  In regular expression patterns, the
     conversion isn't done, but if the character set modifier would
     otherwise be "/d", it is changed to "/u".

 •   If you use the form "\N{_c_h_a_r_a_c_t_e_r _n_a_m_e}", the charnames module gets
     automatically loaded.  This may not be suitable for the test level
     you are doing.

 If you are testing locales (see perllocale), there are helper functions
 in _t_/_l_o_c___t_o_o_l_s_._p_l to enable you to see what locales there are on the
 current platform.

SSppeecciiaall “"mmaakkee tteesstt"” ttaarrggeettss There are various special make targets that can be used to test Perl slightly differently than the standard “test” target. Not all them are expected to give a 100% success rate. Many of them have several aliases, and many of them are not available on certain operating systems.

 •   test_porting

     This runs some basic sanity tests on the source tree and helps catch
     basic errors before you submit a patch.

 •   minitest

     Run _m_i_n_i_p_e_r_l on _t_/_b_a_s_e, _t_/_c_o_m_p, _t_/_c_m_d, _t_/_r_u_n, _t_/_i_o, _t_/_o_p, _t_/_u_n_i and
     _t_/_m_r_o tests.

     _m_i_n_i_p_e_r_l is a minimalistic perl built to bootstrap building
     extensions, utilties, documentation etc.  It doesn't support dynamic
     loading and depending on the point in the build process will only
     have access to a limited set of core modules.  _m_i_n_i_p_e_r_l is not
     intended for day to day use.

 •   test.valgrind check.valgrind

     (Only in Linux) Run all the tests using the memory leak + naughty
     memory access tool "valgrind".  The log files will be named
     _t_e_s_t_n_a_m_e_._v_a_l_g_r_i_n_d.

 •   test_harness

     Run the test suite with the _t_/_h_a_r_n_e_s_s controlling program, instead of
     _t_/_T_E_S_T.  _t_/_h_a_r_n_e_s_s is more sophisticated, and uses the Test::Harness
     module, thus using this test target supposes that perl mostly works.
     The main advantage for our purposes is that it prints a detailed
     summary of failed tests at the end.  Also, unlike _t_/_T_E_S_T, it doesn't
     redirect stderr to stdout.

     Note that under Win32 _t_/_h_a_r_n_e_s_s is always used instead of _t_/_T_E_S_T, so
     there is no special "test_harness" target.

     Under Win32's "test" target you may use the TEST_SWITCHES and
     TEST_FILES environment variables to control the behaviour of
     _t_/_h_a_r_n_e_s_s.  This means you can say

         nmake test TEST_FILES="op/*.t"
         nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"

 •   test-notty test_notty

     Sets PERL_SKIP_TTY_TEST to true before running normal test.

PPaarraalllleell tteessttss The core distribution can now run its regression tests in parallel on Unix-like and Windows platforms. On Unix, instead of running “make test”, set “TEST_JOBS” in your environment to the number of tests to run in parallel, and run “make test_harness”. On a Bourne-like shell, this can be done as

     TEST_JOBS=3 make test_harness  # Run 3 tests in parallel

 An environment variable is used, rather than parallel make itself,
 because TAP::Harness needs to be able to schedule individual non-
 conflicting test scripts itself, and there is no standard interface to
 "make" utilities to interact with their job schedulers.

 Tests are normally run in a logical order, with the sanity tests first,
 then the main tests of the Perl core functionality, then the tests for
 the non-core modules.  On many-core systems, this may not use the
 hardware as effectively as possible.  By also specifying

  TEST_JOBS=19 PERL_TEST_HARNESS_ASAP=1 make -j19 test_harness

 you signal that you want the tests to finish in wall-clock time as short
 as possible.  After the sanity tests are completed, this causes the
 remaining ones to be packed into the available cores as tightly as we
 know how.  This has its greatest effect on slower, many-core systems.
 Throughput was sped up by 20% on an outmoded 24-core system; less on more
 recent faster ones with fewer cores.

 Note that the command line above added a "-j" parameter to make, so as to
 cause parallel compilation.  This may or may not work on your platform.

RRuunnnniinngg tteessttss bbyy hhaanndd You can run part of the test suite by hand by using one of the following commands from the _t_/ directory:

     ./perl -I../lib TEST list-of-.t-files

 or

     ./perl -I../lib harness list-of-.t-files

 (If you don't specify test scripts, the whole test suite will be run.)

UUssiinngg _t_/_h_a_r_n_e_s_s ffoorr tteessttiinngg If you use “harness” for testing, you have several command line options available to you. The arguments are as follows, and are in the order that they must appear if used together.

     harness -v -torture -re=pattern LIST OF FILES TO TEST
     harness -v -torture -re LIST OF PATTERNS TO MATCH

 If "LIST OF FILES TO TEST" is omitted, the file list is obtained from the
 manifest.  The file list may include shell wildcards which will be
 expanded out.

 •   -v

     Run the tests under verbose mode so you can see what tests were run,
     and debug output.

 •   -torture

     Run the torture tests as well as the normal set.

 •   -re=PATTERN

     Filter the file list so that all the test files run match PATTERN.
     Note that this form is distinct from the --rree LLIISSTT OOFF PPAATTTTEERRNNSS form
     below in that it allows the file list to be provided as well.

 •   -re LIST OF PATTERNS

     Filter the file list so that all the test files run match
     /(LIST|OF|PATTERNS)/.  Note that with this form the patterns are
     joined by '|' and you cannot supply a list of files, instead the test
     files are obtained from the MANIFEST.

 You can run an individual test by a command similar to

     ./perl -I../lib path/to/foo.t

 except that the harnesses set up some environment variables that may
 affect the execution of the test:

• PERL_CORE=1 #

     indicates that we're running this test as part of the perl core test
     suite.  This is useful for modules that have a dual life on CPAN.

• PERL_DESTRUCT_LEVEL=2 #

     is set to 2 if it isn't set already (see "PERL_DESTRUCT_LEVEL" in
     perlhacktips).

• PERL #

     (used only by _t_/_T_E_S_T) if set, overrides the path to the perl
     executable that should be used to run the tests (the default being
     _._/_p_e_r_l).

• PERL_SKIP_TTY_TEST #

     if set, tells to skip the tests that need a terminal.  It's actually
     set automatically by the Makefile, but can also be forced
     artificially by running 'make test_notty'.

 _O_t_h_e_r _e_n_v_i_r_o_n_m_e_n_t _v_a_r_i_a_b_l_e_s _t_h_a_t _m_a_y _i_n_f_l_u_e_n_c_e _t_e_s_t_s

 •   PERL_TEST_Net_Ping

     Setting this variable runs all the Net::Ping modules tests, otherwise
     some tests that interact with the outside world are skipped.  See
     perl58delta.

• PERL_TEST_NOVREXX #

     Setting this variable skips the vrexx.t tests for OS2::REXX.

• PERL_TEST_NUMCONVERTS #

     This sets a variable in op/numconvert.t.

• PERL_TEST_MEMORY #

     Setting this variable includes the tests in _t_/_b_i_g_m_e_m_/.  This should
     be set to the number of gigabytes of memory available for testing,
     eg.  "PERL_TEST_MEMORY=4" indicates that tests that require 4GiB of
     available memory can be run safely.

 See also the documentation for the Test and Test::Harness modules, for
 more environment variables that affect testing.

PPeerrffoorrmmaannccee tteessttiinngg The file _t_/_p_e_r_f_/_b_e_n_c_h_m_a_r_k_s contains snippets of perl code which are intended to be benchmarked across a range of perls by the _P_o_r_t_i_n_g_/_b_e_n_c_h_._p_l tool. If you fix or enhance a performance issue, you may want to add a representative code sample to the file, then run _b_e_n_c_h_._p_l against the previous and current perls to see what difference it has made, and whether anything else has slowed down as a consequence.

 The file _t_/_p_e_r_f_/_o_p_c_o_u_n_t_._t is designed to test whether a particular code
 snippet has been compiled into an optree containing specified numbers of
 particular op types. This is good for testing whether optimisations which
 alter ops, such as converting an "aelem" op into an "aelemfast" op, are
 really doing that.

 The files _t_/_p_e_r_f_/_s_p_e_e_d_._t and _t_/_r_e_/_s_p_e_e_d_._t are designed to test things
 that run thousands of times slower if a particular optimisation is broken
 (for example, the utf8 length cache on long utf8 strings).  Add a test
 that will take a fraction of a second normally, and minutes otherwise,
 causing the test file to time out on failure.

BBuuiillddiinngg ppeerrll aatt oollddeerr ccoommmmiittss In the course of hacking on the Perl core distribution, you may have occasion to configure, build and test perl at an old commit. Sometimes “make” will fail during this process. If that happens, you may be able to salvage the situation by using the Devel::PatchPerl library from CPAN (not included in the core) to bring the source code at that commit to a buildable state.

 Here's a real world example, taken from work done to resolve perl #10118
 <https://github.com/Perl/perl5/issues/10118>.  Use of _P_o_r_t_i_n_g_/_b_i_s_e_c_t_._p_l
 had identified commit "ba77e4cc9d1ceebf472c9c5c18b2377ee47062e6" as the
 commit in which a bug was corrected.  To confirm, a P5P developer wanted
 to configure and build perl at commit "ba77e4c^" (presumably "bad") and
 then at "ba77e4c" (presumably "good").  Normal configuration and build
 was attempted:

     $ sh ./Configure -des -Dusedevel
     $ make test_prep

 "make", however, failed with output (excerpted) like this:

     cc -fstack-protector -L/usr/local/lib -o miniperl \
       gv.o toke.o perly.o pad.o regcomp.o dump.o util.o \
       mg.o reentr.o mro.o hv.o av.o run.o pp_hot.o sv.o \
       pp.o scope.o pp_ctl.o pp_sys.o doop.o doio.o regexec.o \
       utf8.o taint.o deb.o universal.o globals.o perlio.o \
       numeric.o mathoms.o locale.o pp_pack.o pp_sort.o  \
       miniperlmain.o opmini.o perlmini.o
     pp.o: In function `Perl_pp_pow':
     pp.c:(.text+0x2db9): undefined reference to `pow'
     ...
     collect2: error: ld returned 1 exit status
     makefile:348: recipe for target 'miniperl' failed
     make: *** [miniperl] Error 1

 Another P5P contributor recommended installation and use of
 Devel::PatchPerl for this situation, first to determine the version of
 perl at the commit in question, then to patch the source code at that
 point to facilitate a build.

  $ perl -MDevel::PatchPerl -e \
      'print Devel::PatchPerl->determine_version("/path/to/sourcecode"),
             "\n";'
  5.11.1
  $ perl -MDevel::PatchPerl -e \
      'Devel::PatchPerl->patch_source("5.11.1", "/path/to/sourcecode");'

 Once the source was patched, "./Configure" and "make test_prep" were
 called and completed successfully, enabling confirmation of the findings
 in RT #72414.

MMOORREE RREEAADDIINNGG FFOORR GGUUTTSS HHAACCKKEERRSS #

 To hack on the Perl guts, you'll need to read the following things:

 •   perlsource

     An overview of the Perl source tree.  This will help you find the
     files you're looking for.

 •   perlinterp

     An overview of the Perl interpreter source code and some details on
     how Perl does what it does.

 •   perlhacktut

     This document walks through the creation of a small patch to Perl's C
     code.  If you're just getting started with Perl core hacking, this
     will help you understand how it works.

 •   perlhacktips

     More details on hacking the Perl core.  This document focuses on
     lower level details such as how to write tests, compilation issues,
     portability, debugging, etc.

     If you plan on doing serious C hacking, make sure to read this.

 •   perlguts

     This is of paramount importance, since it's the documentation of what
     goes where in the Perl source.  Read it over a couple of times and it
     might start to make sense - don't worry if it doesn't yet, because
     the best way to study it is to read it in conjunction with poking at
     Perl source, and we'll do that later on.

     Gisle Aas's "illustrated perlguts", also known as _i_l_l_g_u_t_s, has very
     helpful pictures:

     <https://metacpan.org/release/RURBAN/illguts-0.49>

 •   perlxstut and perlxs

     A working knowledge of XSUB programming is incredibly useful for core
     hacking; XSUBs use techniques drawn from the PP code, the portion of
     the guts that actually executes a Perl program.  It's a lot gentler
     to learn those techniques from simple examples and explanation than
     from the core itself.

 •   perlapi

     The documentation for the Perl API explains what some of the internal
     functions do, as well as the many macros used in the source.

 •   _P_o_r_t_i_n_g_/_p_u_m_p_k_i_n_._p_o_d

     This is a collection of words of wisdom for a Perl porter; some of it
     is only useful to the pumpkin holders, but most of it applies to
     anyone wanting to go about Perl development.

CCPPAANN TTEESSTTEERRSS AANNDD PPEERRLL SSMMOOKKEERRSS #

 The CPAN testers ( <http://cpantesters.org/> ) are a group of volunteers
 who test CPAN modules on a variety of platforms.

 Perl Smokers ( <https://www.nntp.perl.org/group/perl.daily-build/> and
 <https://www.nntp.perl.org/group/perl.daily-build.reports/> )
 automatically test Perl source releases on platforms with various
 configurations.

 Both efforts welcome volunteers.  In order to get involved in smoke
 testing of the perl itself visit
 <https://metacpan.org/release/Test-Smoke>.  In order to start smoke
 testing CPAN modules visit
 <https://metacpan.org/release/CPANPLUS-YACSmoke> or
 <https://metacpan.org/release/minismokebox> or
 <https://metacpan.org/release/CPAN-Reporter>.

WWHHAATT NNEEXXTT?? #

 If you've read all the documentation in the document and the ones listed
 above, you're more than ready to hack on Perl.

 Here's some more recommendations

 •   Subscribe to perl5-porters, follow the patches and try and understand
     them; don't be afraid to ask if there's a portion you're not clear on
     - who knows, you may unearth a bug in the patch...

 •   Do read the README associated with your operating system, e.g.
     README.aix on the IBM AIX OS.  Don't hesitate to supply patches to
     that README if you find anything missing or changed over a new OS
     release.

 •   Find an area of Perl that seems interesting to you, and see if you
     can work out how it works.  Scan through the source, and step over it
     in the debugger.  Play, poke, investigate, fiddle! You'll probably
     get to understand not just your chosen area but a much wider range of
     _p_e_r_l's activity as well, and probably sooner than you'd think.

“"TThhee RRooaadd ggooeess eevveerr oonn aanndd oonn,, ddoowwnn ffrroomm tthhee ddoooorr wwhheerree iitt bbeeggaann..“” If you can do these things, you’ve started on the long road to Perl porting. Thanks for wanting to help make Perl better - and happy hacking!

MMeettaapphhoorriicc QQuuoottaattiioonnss If you recognized the quote about the Road above, you’re in luck.

 Most software projects begin each file with a literal description of each
 file's purpose.  Perl instead begins each with a literary allusion to
 that file's purpose.

 Like chapters in many books, all top-level Perl source files (along with
 a few others here and there) begin with an epigrammatic inscription that
 alludes, indirectly and metaphorically, to the material you're about to
 read.

 Quotations are taken from writings of J.R.R. Tolkien pertaining to his
 Legendarium, almost always from _T_h_e _L_o_r_d _o_f _t_h_e _R_i_n_g_s.  Chapters and page
 numbers are given using the following editions:

 •   _T_h_e _H_o_b_b_i_t, by J.R.R. Tolkien.  The hardcover, 70th-anniversary
     edition of 2007 was used, published in the UK by Harper Collins
     Publishers and in the US by the Houghton Mifflin Company.

 •   _T_h_e _L_o_r_d _o_f _t_h_e _R_i_n_g_s, by J.R.R. Tolkien.  The hardcover,
     50th-anniversary edition of 2004 was used, published in the UK by
     Harper Collins Publishers and in the US by the Houghton Mifflin
     Company.

 •   _T_h_e _L_a_y_s _o_f _B_e_l_e_r_i_a_n_d, by J.R.R. Tolkien and published posthumously
     by his son and literary executor, C.J.R. Tolkien, being the 3rd of
     the 12 volumes in Christopher's mammoth _H_i_s_t_o_r_y _o_f _M_i_d_d_l_e _E_a_r_t_h.
     Page numbers derive from the hardcover edition, first published in
     1983 by George Allen & Unwin; no page numbers changed for the special
     3-volume omnibus edition of 2002 or the various trade-paper editions,
     all again now by Harper Collins or Houghton Mifflin.

 Other JRRT books fair game for quotes would thus include _T_h_e _A_d_v_e_n_t_u_r_e_s
 _o_f _T_o_m _B_o_m_b_a_d_i_l, _T_h_e _S_i_l_m_a_r_i_l_l_i_o_n, _U_n_f_i_n_i_s_h_e_d _T_a_l_e_s, and _T_h_e _T_a_l_e _o_f _t_h_e
 _C_h_i_l_d_r_e_n _o_f _H_u_r_i_n, all but the first posthumously assembled by CJRT.  But
 _T_h_e _L_o_r_d _o_f _t_h_e _R_i_n_g_s itself is perfectly fine and probably best to quote
 from, provided you can find a suitable quote there.

 So if you were to supply a new, complete, top-level source file to add to
 Perl, you should conform to this peculiar practice by yourself selecting
 an appropriate quotation from Tolkien, retaining the original spelling
 and punctuation and using the same format the rest of the quotes are in.
 Indirect and oblique is just fine; remember, it's a metaphor, so being
 meta is, after all, what it's for.

AAUUTTHHOORR #

 This document was originally written by Nathan Torkington, and is
 maintained by the perl5-porters mailing list.

perl v5.36.3 2023-02-15 PERLHACK(1)